When the drive of a lubricant pump (46) is stopped to prevent the lubricant pump (46) from assuming an idling state, a determination is made as to whether or not a temperature difference (ΔT) between the temperature (T) of a lubricant oil detected earlier by a temperature sensor (51) and a warning temperature (Toh) is equal to or lower than a predetermined threshold value (α). When the temperature difference (ΔT) is within the threshold value (α), an “overheat” warning is issued to an operator of a dump truck (1). In consequence, even in a case where a temperature sensor (51) is provided on the output side of the lubricant pump (46) by being located on an outer side of a wheel mounting case (19), planetary gear reduction mechanisms (23), (31) inside the wheel mounting case (19) can be prevented from overheating by warning the operator of the vehicle.